Present work machines, such as wheel loaders, may utilize a hydrostatic transmission to assist in controlling ground speed. Typically, a plurality of variable displacement motors that drive the wheels of the work machine are displaced by a pressure signal within a signal line that is generated from a fixed displacement pump linked directly with an engine. Therefore, the ground speed is engine speed dependent so that as the engine speed increases the signal pressure increases to further displace the motor, increasing the ground speed. However, a "ball valve" is located in the signal line that can be opened to bleed off a volume of hydraulic fluid from the signal line to a tank, which subsequently reduces the signal pressure controlling the motor, slowing or even stopping the machine. Generally, this is referred to as the neutralization of the hydrostatic transmission so that the ground speed of the work machine is controlled substantially independent of the engine speed, allowing for increased power availability to the implement. However, during neutralization of the hydrostatic transmission, it is important to reach an engine speed that is high enough to operate an implement and maintain that engine speed throughout various loading conditions. Generally, this requires that an operator must continuously monitor and compensate for changes in the engine speed during operation of the implement, such as through the manual control of a governor pedal.
An approach for simultaneously controlling ground and engine speeds is disclosed in U.S. Pat. No. 4,157,124 issued to Bernard B. Poore on Jun. 5, 1979. This patent utilizes a vehicular gas turbine engine power system with an engine, an automatic clutch, a service clutch and an infinitely variable transmission coupled along a power train. A control system is utilized to control and vary the engine speed for maximum fuel efficiency in response to operator selected ground and engine speed commands. The engine speed is controlled substantially through exhaust gas temperature and engine speed feedback, reducing the ground speed, if necessary, to obtain the desired engine speed. The control system disclosed utilizes an advanced system for engine and ground speed control that functions with an infinitely variable transmission. The use of the disclosed control system would not be feasible on a work machine utilizing a hydrostatic transmission because of the need to control the engine speed independently from the ground speed. Preferably, the ability to control the engine speed independently from the ground speed control would utilize a separate electronic engine speed controller. Therefore, the engine could be set at any desired speed independent of the ground speed to efficiently drive a hydraulic implement pump for operating the implement and maintain the desired speed substantially throughout the implement operation. This ability would ensure efficient use of the engine, increase operator flexibility and control, and minimize operator fatigue. The engine speed controller should be relatively simple and inexpensive so that it may be easily added to an existing work machine already utilizing the hydrostatic transmission for controlling ground speed.
The present invention is directed to overcoming the problems as set forth above.